Calculate body density from skinfold measurements and convert to body fat percentage using the Siri and Brozek equations. Free online tool.
The Body Density Calculator converts skinfold thickness measurements into whole-body density using the Jackson–Pollock generalized equations, then translates that density into body fat percentage with both the Siri and Brozek conversion formulas. Body density is expressed in g/cm³ and serves as the intermediate step in many body-composition assessment methods, including skinfold calipers, hydrostatic (underwater) weighing, and air-displacement plethysmography (Bod Pod).
Knowing your body density is valuable because it bridges the gap between a simple skinfold measurement and a clinically meaningful body fat estimate. Researchers and coaches use density-based methods because they are validated against criterion techniques like DEXA, making them more trustworthy than circumference-only estimators.
This calculator supports both the 3-site and 7-site Jackson–Pollock protocols, with sex-specific site selections, and displays results from both the Siri and Brozek equations so you can compare and choose the estimate that best fits your population or preference. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
Body density is the gold standard intermediate measurement in body-composition science. Converting skinfold data to density and then to body fat percentage follows the same workflow used in academic labs and sports-science clinics. By seeing the density value itself — not just the final body fat number — you gain deeper insight into the method's accuracy, can compare results across different protocols, and can verify your measurements against published norms.
Jackson–Pollock 3-Site (Male — chest, abdomen, thigh): Body Density = 1.10938 − 0.0008267 × S + 0.0000016 × S² − 0.0002574 × Age where S = sum of 3 skinfolds in mm. 3-Site (Female — tricep, suprailiac, thigh): Body Density = 1.0994921 − 0.0009929 × S + 0.0000023 × S² − 0.0001392 × Age 7-Site (Male — chest, midaxillary, tricep, subscapular, abdomen, suprailiac, thigh): Body Density = 1.112 − 0.00043499 × S + 0.00000055 × S² − 0.00028826 × Age 7-Site (Female — same 7 sites): Body Density = 1.097 − 0.00046971 × S + 0.00000056 × S² − 0.00012828 × Age Siri Equation: BF% = (495 / Density) − 450 Brozek Equation: BF% = (457 / Density) − 414.2
Result: Density 1.0640 g/cm³ → Siri 15.2% BF, Brozek 15.3% BF
A 30-year-old male measures chest 12 mm, abdomen 20 mm, and thigh 15 mm. Sum = 47 mm. Jackson–Pollock 3-site male: Density = 1.10938 − 0.0008267×47 + 0.0000016×47² − 0.0002574×30 = 1.0640 g/cm³. Siri: BF% = (495/1.0640) − 450 = 15.2%. Brozek: BF% = (457/1.0640) − 414.2 = 15.3%. Both equations agree closely for this individual, who falls in the fitness range for his age.
Body density has been used in body-composition research since the 1940s, when Albert Behnke first applied Archimedes' principle to human underwater weighing. The concept is simple: fat tissue is less dense than water (about 0.9 g/cm³), while lean tissue is denser (about 1.1 g/cm³). A person's overall density therefore indicates their relative proportion of fat to lean tissue.
In 1978 and 1980, Andrew Jackson and Michael Pollock published generalized regression equations that predict body density from skinfold thicknesses, age, and sex. Unlike earlier population-specific models, these equations were validated across a wide range of body types and ages, making them suitable for general use. The 3-site and 7-site protocols remain the most widely cited skinfold methods in exercise science.
The Siri equation (1961) and the Brozek equation (1963) are the two principal formulas for converting density to body fat. Both assume a two-component model (fat vs. fat-free mass), though Brozek's constants are derived from slightly different tissue-density assumptions. For most healthy adults the difference is less than 0.5 percentage points, but in populations with atypical bone density or hydration — such as the elderly, children, or certain ethnic groups — more advanced multi-component models are recommended.
Caliper technique is the single biggest source of error in skinfold-based density estimation. Practitioners should train on at least 50 practice measurements before collecting data for research. In self-assessment, mark measurement sites with a pen to ensure consistency, measure in the morning before exercise, and avoid skin lotions that can make folds slippery.
Body density is total body mass divided by total body volume, typically expressed in g/cm³. Lean tissue is denser (~1.1 g/cm³) than fat tissue (~0.9 g/cm³), so a higher body density indicates a leaner body. Density is the intermediary value needed to convert skinfold or hydrostatic-weighing data into body fat percentage.
Both convert body density to body fat percentage, but they use slightly different models of the body's fat and fat-free mass densities. The Siri equation assumes fat-free mass density of 1.1 g/cm³; Brozek uses a four-component model. For most adults the results are very close. Brozek tends to be more accurate at the extremes — very lean or very high body fat.
The 7-site protocol samples more body areas and is generally considered more accurate, but requires more skill and time. The 3-site protocol is quicker and still well-validated. For periodic self-tracking, 3-site is practical. For clinical or research purposes, 7-site is preferred.
Yes, though Harpenden and Lange calipers are used in most validation studies. Less expensive calipers can work for tracking trends, but absolute values may differ by 1–2 mm. Consistency — using the same caliper each time — is more important than brand choice.
Men and women store fat in different patterns. The Jackson–Pollock equations were developed using sex-specific regression models that select the sites most predictive of total body density for each sex. Using the wrong formula can introduce notable errors.
The equations include an age term because body density tends to decrease with age, even at the same skinfold thickness, due to changes in internal (visceral) fat, bone density, and hydration. Omitting age would overestimate density (underestimate body fat) in older adults.
Healthy adult males typically have body densities between 1.04 and 1.08 g/cm³ (roughly 8–25% body fat). Healthy adult females range from 1.02 to 1.06 g/cm³ (roughly 18–32% body fat). Elite athletes can exceed 1.08 g/cm³ for males and 1.06 g/cm³ for females.
Hydrostatic weighing measures body density directly by comparing dry weight to underwater weight. Skinfold-based body density is a prediction equation that estimates the density a hydrostatic test would produce. The two methods should agree within 1–2% body fat for most individuals when skinfold technique is good.